The density characteristics of images printed by an image forming apparatus vary under the influence of factors such as change in the characteristics of components over time, variation in characteristics at the time of manufacture, and the use environment. Japanese Patent Laid-Open No. 2008-249714 discloses a configuration for adjusting density by forming a patch image for detecting density.
In Japanese Patent Laid-Open No. 2008-249714, first, light is irradiated by a light emitting element consisting of an infrared light emitting diode or the like onto a color toner image formed on an intermediate transfer body, and light that is specular reflected at this time is received by a light receiving element for specular reflection, while light that is diffuse reflected is received by a light receiving element for diffuse reflection. Here, the light receiving elements can be constituted by phototransistors, for example. The density of the color toner image is derived from the output of both light receiving elements.
At this time, the infrared light emitting diode and phototransistors are held by being enclosed in packages. Passageways are formed in these packages for securing a light path for light irradiated by the light emitting element to travel to the object being irradiated, and a light path for light specularly reflected by the object being irradiated to travel to the light receiving elements. A passageway for securing a light path for light diffusely reflected by the object being irradiated to travel to the light receiving elements may also be formed in the packages.
With conventionally known sensors for detecting the light quantity of a patch image, it is, for instance, necessary to form light passageways in the packages, as described above, in order to separate specular reflected light and diffuse reflected light, with this being a problem in that it leads to an increase in size of the light quantity detection sensor.